1. The Field of the Invention
The present invention relates to structural diaphragm walls constructed by reinforced soilcrete columns and processes for constructing such walls. More particularly, the present invention utilizes load bearing members, such as a steel I-beam, in combination with soilcrete columns of various diameters to produce a structural diaphragm wall more efficiently and stronger than the currently used techniques in the art.
2. Related Application
This patent application is a continuation-in-part of copending U.S. patent application Ser. No. 07/172,286, filed Mar. 23, 1988, in the name of Osamu Taki, and entitled "MULTI-SHAFT AUGER APPARATUS AND PROCESS FOR FIXATION OF SOILS CONTAINING TOXIC WATERS," which patent application is incorporated herein by specific reference.
3. The Prior Art
For a number of years, auger machines have been used in Japan to construct concrete-like columns in the ground without having to excavate the soil. These columns are sometimes referred to as "soilcrete" columns, because the soil is mixed with a cement hardener in situ. Upon hardening, the soilcrete columns possess characteristics of concrete columns, but they are constructed without the expense and time consuming processes of removing and replacing the soil with concrete.
To produce soilcrete columns, a multi-shaft auger machine bores holes in the ground and simultaneously mixes the soil with a chemical hardening material pumped from the surface through the auger shaft to the end of the auger. Multiple columns are prepared while the soil-cement mixture is still soft in order to form continuous walls or geometric patterns within the soil, the particular shape or pattern depending on the purpose of the soilcrete columns.
Because the soil is mixed in situ and because the soilcrete wall is formed in a single process step, the construction period is shorter than for other construction methods. Obviously, the costs of forming soilcrete columns are less than traditional methods requiring excavation of the soil in order to form concrete pillars or walls. In addition, because the soil is not removed from the ground, there is comparatively little material (produced by such in situ processes) that must be disposed of during the course of construction.
The soilcrete columns have been arranged in a variety of patterns depending on the desired application. Soilcrete columns are used to improve the load bearing capacity of soft soils, such as sandy or soft clay soils. The columns are formed deep in the ground and form a solid base or "foundation" for anchoring or supporting surface construction on such soft soils. Soilcrete columns have been overlapped to form boundary walls, low to medium capacity soil-mixed caissons, and piles which act as a base for construction.
One application for boundary walls constructed of soilcrete columns which has been found to be particularly useful is as a pressure retaining wall, such as a wall which is subject to hydrostatic pressure. For example, walls constructed of soilcrete columns have been found to be particularly useful in underground construction work when ground water levels interfere with construction. Such structural boundary walls also find useful application as a retaining wall to provide lateral support for ground adjacent sites of excavation.
For example, the construction of large buildings in metropolitan areas often requires deep excavation immediately adjacent a street or plaza which would be in danger of crumbling unless some precaution were taken to provide lateral support to these areas. By first constructing a structural boundary wall, excavation may take place without the danger of injury to property adjacent the construction site.
Construction of subways, underground parking facilities, and sewage tunnels may also be facilitated by the use of soilcrete boundary walls to assist in controlling both the ground water problem and to provide lateral support to land adjacent the construction site. Indeed, boundary walls constructed from soilcrete columns have been found particularly useful in many applications where the wall is subject to hydrostatic pressure or pressure from other sources.
Although soilcrete columns do not typically contain cement, the strength characteristics of such columns are very similar to columns made of cement. Such columns tend to be very strong in compression but, because of their brittle nature, are relatively weak when subjected to forces which place any portion of the columns in tension.
When a wall constructed from soilcrete columns is used in a structural boundary wall application, it is subjected to lateral pressure on one side. The forces imparted on the wall place a bending moment on the columns, thereby subjecting one side of the columns to tensile stresses.
If the tensile stresses are of sufficient magnitude, brittle fractures will occur in the columns, thereby reducing the strength of the columns and possibly even causing the columns to fail. Such a result could have disastrous consequences particularly when the columns are used as a structural foundation for large buildings or other applications wherein human life could be placed in jeopardy by the failure of these columns.
One solution for strengthening soilcrete columns which has been proposed by the prior art is simply to build walls of columns having a greater diameter. Building walls comprised of larger diameter columns, however, requires larger, more expensive equipment and increased labor, resulting in a substantial (and a potentially prohibitive) increase in the costs of construction.
An even more fundamental problem with building larger diameter columns is that the morphology of the columns is not changed by increasing the diameter of the columns; the columns are still brittle and unable to withstand large tensile stresses. If subjected to a large enough tensile force, the same problems will occur. Building walls made up of larger diameter columns, therefore, has found to be an unacceptable solution to the problem.
An alternative solution which has been proposed by the prior art is to place a load bearing member, such as a steel I-beam, within the columns, thereby forming a composite column. This proposed solution is depicted in FIG. 1. When such a composite column is subjected to the bending moments described above, thereby placing certain portions of the column in tension, the reinforcing member, which is very strong in tension, bears those loads. A composite column of this nature thus retains the advantage of being strong in compression, thereby enabling it to be used in a foundation application, while also having increased tensile properties over non-composite soilcrete columns as is required if the columns are to be used to form a boundary wall.
A structural diaphragm wall constructed of composite soilcrete columns as described above, however, also has some disadvantages. It will be appreciated that the cost of placing an I-beam of similar structural member within each of the soilcrete columns can be extremely expensive. Construction of soilcrete columns requires large, expensive machinery. Additionally, the construction process is highly labor intensive. If a structural member is to be placed within each column which forms the diaphragm wall, the high costs of labor and materials can effectively outweigh the other advantages associated with in situ soilcrete column construction.
It has been found that the extra cost associated with placing a structural member in each column can be drastically decreased while maintaining the structural advantages of the composite columns described above by placing a reinforcing member only in alternating columns. Such a prior art configuration is illustrated in FIG. 2. This configuration reduces by one-half the number of reinforcing members needed to construct a wall. The approach of FIG. 2 results not only in a cost savings for materials, but also decreases significantly the amount of labor required to construct a reinforced structural diaphragm wall.
Although the strength and load-bearing capacity supplied by the structural members is reduced because of the elimination of some of the structural members, it has been found that the configuration of FIG. 2 has sufficient strength for some applications. FIGS. 3, 4, and 5 also illustrate various configurations employed by the prior art in an attempt to reduce labor, construction time, and material cost while maintaining a structural diaphragm wall with the strength characteristics necessary to withstand the pressures imposed on the wall.
It will be appreciated by those skilled in the art, nevertheless, that a soilcrete wall configured as illustrated in any of FIGS. 2 through 5 will not have nearly the same strength in tension as a wall configured as illustrated in FIG. 1.
Although walls constructed with alternating composite columns may be constructed at a lower cost than walls using all composite columns, such soilcrete walls may still be prohibitively expensive thereby rendering their use impractical for many applications. As a stronger wall is required, larger diameter auger equipment must be used to drill soilcrete columns having a larger diameter thereby enabling a larger structural member to be placed within the column to provide the additional required strength. Construction costs thus increase dramatically as the required strength of the wall increases.
From the foregoing, it will be appreciated that what is needed in the art are in situ reinforced structural diaphragm walls and methods of manufacturing such walls in which the structural advantages of walls comprised of soilcrete columns containing reinforcing members, such as steel I-beams, in alternating columns are utilized and which may be constructed such that both the capital and the operating costs associated with such construction are lower than those costs associated with the structural diaphragm walls of the prior art.
It would also be an advancement in the art if such walls could be provided which have load bearing capabilities equivalent or superior to those of prior art walls but which can be constructed according to a more efficient design whereby the load bearing capacity of the wall is enhanced and the time and the costs of construction are reduced.
Such walls and methods for construction are disclosed and claimed herein.